The present invention relates to a process for manufacturing lithium hexafluorophosphate (LiPF6).
Lithium hexafluorophosphate is used mainly as an electrolyte in rechargeable batteries.
In this application, the current specifications regarding the weight contents of the main impurities are as follows:
hydrogen fluoride (HF) less than 200 ppm
lithium fluoride (LiF) less than 1000 ppm.
The contents of phosphorus oxyfluorides (O=PF3, O=PF2OH) are not explicitly indicated, but it is considered that their sum should be less than 1000 ppm.
In the rechargeable batteries industry, it has been noted that traces of acid impurities (labile protons H+) are harmful to the electrochemical behaviour of the battery, in particular to the formation of the passivation layer at the interface of the lithium electrode/electrolyte or LixC6/electrolyte, which has the consequence of adversely affecting the capacitance of the battery and its lifetime (number of charging and discharging cycles).
The problem arises of industrially obtaining LiPF6 with a very low content of acid impurities in order to obtain a high-quality electrolyte.
Virtually all of the known processes for synthesizing LiPF6 involve a step in which the crystalline LiPF6 is either dissolved or placed in contact with a large excess of liquid HF. The strategy then consists in obtaining, by crystallization, large (1 to 3 mm) LiPF6 crystals tending towards being as perfect as possible (quasi-monocrystals) in order to minimize the amounts of HF which are dissolved and/or occluded therein.
Thus, in Japanese patent application 60-251 109, PCl5 is added to a solution of LiF in liquid anhydrous HF in order to obtain LiPF6 in solution, and the HF is then evaporated off slowly. The product obtained is in the form of crystals 1 to 3 mm in size, accompanied by a white powder containing LiF. Example 1 indicates the production of 40 g of LiPF6 in a yield of 65.1% and a purity of at least 99%. However, the methods for the analytical determination of this purity are not indicated.
Japanese patent application 4-175 216 teaches the production of gaseous PF5 by reacting PCl5 with anhydrous HF in large excess at xe2x88x9230xc2x0 C. to give firstly white crystals of HPF6 in HF, and the concomitant evaporation of the hydrogen chloride (HCl) produced in the reaction. The temperature is then raised to xe2x88x9210xc2x0 C. to give gaseous PF5 according to the reaction:
HPF6xe2x86x92PF5+HF
The gaseous PF5 thus obtained is introduced continuously into a solution of LiF.HF in HF in order to obtain LiPF6 crystals with a uniform diameter of 2 to 3 mm. These crystals are separated from the filtrate by an ordinary technique which is not specified. The yield is 70% and the purity indicated is 99.98%. The analytical methods indicated for the lithium and phosphorus are, respectively, atomic absorption spectrometry and absorptiometry. On the other hand, the HF assay method is not indicated.
Japanese patent application 5-279 003 discloses a process for synthesizing LiPF6 which can be used as an electrolyte in rechargeable batteries and which contains no phosphorus oxyfluoride (O=PF3). PCl5 is reacted with HF to give a gaseous mixture of PF5 and HCl. This mixture is cooled to between xe2x88x9240xc2x0 C. and xe2x88x9284xc2x0 C. in order to remove the O=PF3 by separation, and is then introduced into a solution of LiF in HF to form LiPF6. The HF is distilled off under nitrogen in order to concentrate the solution, and this solution is then cooled to between 0xc2x0 C. and xe2x88x9220xc2x0 C. to give LiPF6 crystals. These crystals are separated out by filtration and then flushed with nitrogen at between 40xc2x0 C. and 80xc2x0 C. to remove the residual HF. Examples 1, 2 and 3 indicate respective yields of 65.6%, 62.3% and 64.9% crystalline LiPF6 having respective residual contents of acid compounds, assayed by titrimetry in aqueous medium, of 11, 12 and 12 ppm of H+, which corresponds for a person skilled in the art to 220, 240 and 240 ppm of HF.
Japanese patent application 6-56413 discloses a process comprising the following steps:
A PF5+5HCl gaseous mixture is generated by reacting solid PCl5 with HF gas at between 60 and 165xc2x0 C. The gaseous mixture thus obtained is introduced into a reactor-crystallizing vessel containing LiF dissolved in liquid HF. The crystals obtained are dissolved by raising the temperature to ambient temperature and the solution obtained is then crystallized by lowering the temperature to xe2x88x9220xc2x0 C. to give crystals with a particle size of 1 to 3 mm. The crystals are collected and then dried under reduced pressure. Example 1 indicates a yield of 35% for a purity of at least 99%. The mother liquors from Example 1 which still contain 113 g of LiPF6 are recycled in Example 2 with a fresh amount of LiF to give LiPF6 in a yield of 95% calculated relative to the fresh amount of LiF and an indicated purity of at least 99%. The analysis methods for establishing the purity are not mentioned.
Japanese patent application 6-298 506 discloses a process for recrystallizing LiPF6 in HF, comprising a final step of drying under vacuum or under a stream of dry nitrogen at between 60xc2x0 C. and 130xc2x0 C. The residual content of acid impurities indicated in Examples 1, 2 and 3 is, respectively, 150, 100 and 60 ppm, which is interpreted by a person skilled in the art as being an HF content. The method for determining these contents is not indicated.
Japanese patent application 9-268 005 discloses a process for crystallizing LiPF6 in HF, comprising a final step. of drying. Examples 1 and 3 indicate respective contents of free acid of 100 and 99 ppm for yields which are not indicated. To a person skilled in the art, the free acid is HF. The contents of insoluble residues in these same examples are, respectively, 0.08% and 0.07% by weight. In addition, the method for determining these contents is not dscribed.
In summary, since the above prior art does not describe the methods for analysing the residual contents of HF and of phosphorus oxyfluorides, it does not allow a person skilled in the art to check by experimentation the values indicated.
Moreover, patent application WO 98/06666 discloses a process in which LiF is reacted with PCl5 or POCl3 at a reaction temperature of from xe2x88x9220xc2x0 C. to 300xc2x0 C. for 0.1 to 10 h to give LiPF6 according to:
a) PCl5+6LiFxe2x86x925LiCl+LiPF6 
b) 4POCl3+18LiFxe2x86x9212LiCl+Li3PO4+3LiPF6 
and LiPF6 is isolated in solution form from the mixture: ethers, nitriles, esters, sulphones, carbonates, halogenated hydrocarbons and/or tertiary amines are used as solvent.
The purity obtained (Example 1) is only 99.8%, since 0.2% chloride remains in the LiPF6. Furthermore, the analysis methods are not described.
The aim of the present invention is to provide a process for manufacturing LiPF6 with a residual content of HF of less than 20 ppm (lower limit of detection for the analytical method) by reacting LiF and PF5, in a yield, relative to the LiF reacted, of greater than 99%.
The process proposed must also be industrial to allow the manufacture of large amounts of LiPF6 with an HF content of less than 20 ppm.
These objectives are achieved by the process for manufacturing LiPF6 according to the invention, by reacting LiF with PF5 in a liquid, characterized in that this liquid is sulphur dioxide (SO2).
Liquid SO2 replaces liquid HF as reaction medium and thus makes it possible to avoid all the technical problems associated with the removal of HF from the solid LiPF6.
The liquid containing the LiF and PF5 is advantageously at a temperature below 40xc2x0 C.
This temperature preferably ranges from xe2x88x9210xc2x0 C. to +10xc2x0 C.
The PF5/LiF molar ratio is advantageously greater than or equal to 1.05. This excess of PF5 avoids the reverse reaction of decomposition of the LiPF6 into PF5 and LiF.
Advantageously, the PF5 which reacts with LiF in the. liquid SO2 is derived from a gaseous mixture containing hydrogen chloride (HCl). Specifically, HCl behaves as an inert compound, without causing a reduction in the purity of the LiPF6 obtained.
Preferably, from an industrial point of view, the HCl is present in the reaction in an HCl/PF5 molar ratio substantially equal to 5. Specifically, the PF5 used in the industrial synthesis of LiPF6 is often produced by reacting PCl5 with anhydrous HF, leading to a PF5+5HCl gaseous mixture.
The process of the present invention also makes it possible to greatly minimize the other common impurities of LiPF6, in particular phosphorus oxyfluoride compounds, especially O=PF3 and O=PF2OH.
Thus, the present invention makes it possible to obtain solid LiPF6 in a relative weight purity of greater than or equal to 99.90%.
The solid LiPF6 has a residual HF content of less than 20 ppm (detection limit of the analytical method used).
The solid LiPF6 also has a content of 1,2-dimethoxyethane-insoluble residue of less than 100 ppm (detection limit of the analytical method used).
The present invention will be understood more clearly with the aid of the experimental section which follows.
Experimental Section
1/Analytical Methods
a) Assay of the traces of HF, O=PF3 and O=PF2OH.
These assays are carried out by 19F NMR.
The sample of solid to be analysed is transferred into a dry glove box (the ambient H2O content is less than 10 ppm by volume).
A known amount of LiPF6 powder is taken and deuterated acetonitrile (CD3CN, 0.5 ml) predried over 3 xc3x85 molecular sieves is added. The powder dissolves, forming a solution. This solution is introduced into a screw-topped NMR tube and the 19F spectrum is then recorded.
The doublet centred at 6 ppm corresponds to the PF6xe2x88x92 species with a coupling constant 1J Pxe2x80x94F of 707 Hz. The doublet centred at xe2x88x92104 ppm corresponds to the HF species with a coupling constant 1J Hxe2x80x94F of 480 Hz. Two other signals are occasionally detected and correspond to the two products of hydrolysis of LiPF6, namely O=PF2OH and O=PF3, whose doublets are respectively centred at 6.4 ppm (1J Pxe2x80x94F=932 Hz) and at 10.6 ppm (1J Pxe2x80x94F=1064 Hz).
The relative HF content with respect to PF6xe2x88x92, O=PF2OH and O=PF3 comes down to measuring the surface areas of the lines corresponding to HF, LiPF6, O=PF2OH and O=PF3. The process is performed in the same way for O=PF2OH and O=PF3.
The calculations are as follows:
      relative molar % of HF    =            (              S        ⁢                  xe2x80x83                ⁢        signal        ⁢                  xe2x80x83                ⁢        at        ⁢                  xe2x80x83                ⁢                  -                ⁢        104        ⁢                  xe2x80x83                ⁢        ppm            )                                            [                                                            (                                      S                    ⁢                                          xe2x80x83                                        ⁢                    signal                    ⁢                                          xe2x80x83                                        ⁢                    at                    ⁢                                          xe2x80x83                                        ⁢                    6.0                    ⁢                                          xe2x80x83                                        ⁢                    ppm                                    )                                /                6                            +                              (                                  S                  ⁢                                      xe2x80x83                                    ⁢                  signal                  ⁢                                      xe2x80x83                                    ⁢                  at                  ⁢                                      xe2x80x83                                    ⁢                                      -                                    ⁢                  104                  ⁢                                      xe2x80x83                                    ⁢                  ppm                                )                            +                                                                                                                        (                                      S                    ⁢                                          xe2x80x83                                        ⁢                    signal                    ⁢                                          xe2x80x83                                        ⁢                    at                    ⁢                                          xe2x80x83                                        ⁢                    6.4                    ⁢                                          xe2x80x83                                        ⁢                    ppm                                    )                                /                2                            +                                                (                                      S                    ⁢                                          xe2x80x83                                        ⁢                    signal                    ⁢                                          xe2x80x83                                        ⁢                    at                    ⁢                                          xe2x80x83                                        ⁢                    10.6                    ⁢                                          xe2x80x83                                        ⁢                    ppm                                    )                                /                3                                      ]                                    relative molar % of O    =                    PF        2            ⁢      OH        =                  (                  S          ⁢                      xe2x80x83                    ⁢          signal          ⁢                      xe2x80x83                    ⁢          at          ⁢                      xe2x80x83                    ⁢          6.4          ⁢                      xe2x80x83                    ⁢          ppm                )                                                        [                                                                    (                                          S                      ⁢                                              xe2x80x83                                            ⁢                      signal                      ⁢                                              xe2x80x83                                            ⁢                      at                      ⁢                                              xe2x80x83                                            ⁢                      6.0                      ⁢                                              xe2x80x83                                            ⁢                      ppm                                        )                                    /                  6                                +                                  (                                      S                    ⁢                                          xe2x80x83                                        ⁢                    signal                    ⁢                                          xe2x80x83                                        ⁢                    at                    ⁢                                          xe2x80x83                                        ⁢                                          -                                        ⁢                    104                    ⁢                                          xe2x80x83                                        ⁢                    ppm                                    )                                +                                                                                                                                              (                                          S                      ⁢                                              xe2x80x83                                            ⁢                      signal                      ⁢                                              xe2x80x83                                            ⁢                      at                      ⁢                                              xe2x80x83                                            ⁢                      6.4                      ⁢                                              xe2x80x83                                            ⁢                      ppm                                        )                                    /                  2                                +                                                      (                                          S                      ⁢                                              xe2x80x83                                            ⁢                      signal                      ⁢                                              xe2x80x83                                            ⁢                      at                      ⁢                                              xe2x80x83                                            ⁢                      10.6                      ⁢                                              xe2x80x83                                            ⁢                      ppm                                        )                                    /                  3                                            ]                                                relative molar % of O    =            PF      3        =                  (                  S          ⁢                      xe2x80x83                    ⁢          signal          ⁢                      xe2x80x83                    ⁢          at          ⁢                      xe2x80x83                    ⁢          10.6          ⁢                      xe2x80x83                    ⁢          ppm                )                                                        [                                                                    (                                          S                      ⁢                                              xe2x80x83                                            ⁢                      signal                      ⁢                                              xe2x80x83                                            ⁢                      at                      ⁢                                              xe2x80x83                                            ⁢                      6.0                      ⁢                                              xe2x80x83                                            ⁢                      ppm                                        )                                    /                  6                                +                                  (                                      S                    ⁢                                          xe2x80x83                                        ⁢                    signal                    ⁢                                          xe2x80x83                                        ⁢                    at                    ⁢                                          xe2x80x83                                        ⁢                                          -                                        ⁢                    104                    ⁢                                          xe2x80x83                                        ⁢                    ppm                                    )                                +                                                                                                                                              (                                          S                      ⁢                                              xe2x80x83                                            ⁢                      signal                      ⁢                                              xe2x80x83                                            ⁢                      at                      ⁢                                              xe2x80x83                                            ⁢                      6.4                      ⁢                                              xe2x80x83                                            ⁢                      ppm                                        )                                    /                  2                                +                                                      (                                          S                      ⁢                                              xe2x80x83                                            ⁢                      signal                      ⁢                                              xe2x80x83                                            ⁢                      at                      ⁢                                              xe2x80x83                                            ⁢                      10.6                      ⁢                                              xe2x80x83                                            ⁢                      ppm                                        )                                    /                  3                                            ]                                          
(S: denoting the surface area of the line corresponding to each of the species).
Under the following analysis conditions, the HF detection limit is 150 mol.ppm relative to the total sum of the fluoro species present in the solution analysed. This limit value expressed in ppm by mass is substantially 20.
19F NMR analysis conditions:
DRX400 Avance Bruker spectrometer
1H/13C/19F 3-channel probe
observation frequency 376.45 MHz
spectral width examined 80 000 Hz
pulse angle 90xc2x0
acquisition time 3.3 s
reference for the chemical shifts (xcex4=0): trifluoroacetic acid (CF3COOH) as external reference.
Sign convention: xe2x88x92 to the right and + to the left.
The quantitativity conditions were obtained by measuring the longitudinal relaxation time T1 of the PF6xe2x88x92 and HF species (reverse-recovery method). The experimental conditions were adapted to take account of a relaxation time of greater than 5 times T1.
b) Gravimetric assay of the insoluble residues in LiPF6.
All the containers used are made of perfluoroalkoxyalkane (PFA). They are washed beforehand with deionized water and then dried for 2 hours at 105xc2x0 C. After drying, they are introduced into a glove box whose controlled atmosphere has a water content of less than 10 ppm by volume. The filter membrane is made of polytetrafluoroethylene (PTFE) and has a mesh size of 0.45 xcexcm. This membrane is washed, dried, weighed (weight a) and then introduced into the glove box. All the following operations are carried out 24 hours after introducing the various containers and the filter membrane into the glove box.
An accurately weighed sample of about 20 g of LiPF6, of weight m, is introduced into a beaker. 300 ml of 1,2-dimethoxyethane (DME) predried over 3 xc3x85 molecular sieves are then added thereto. After dissolving the LiPF6, a solution is obtained, which is filtered through the PTFE membrane. This filtered solution is rinsed with 100 ml of DME. After rinsing the membrane, it is dried at 100xc2x0 C. for 1 hour. After cooling, the membrane is again weighed (weight b).
The content of insoluble material is calculated in the following way:
E(% by weight)=100xc3x97(bxe2x88x92a)/m
The absolute uncertainty on each weight gain measurement is 1 mg, which leads to a relative uncertainty of 100 ppm relative to the LiPF6 sample.
c) Assay of the total sulphur in LiPF6 
The solid to be analysed is transferred into a dry glove box whose atmosphere has a water content of less than 10 ppm by volume.
An accurately weighed sample of about 5 g of LiPF6 is then introduced into an approximately 50 cm3 flask fitted with a stopper. The flask containing the sample is removed from the glove box. About 20 cm3 of deionized water are introduced into the flask to dissolve the sample, followed by addition of 5 cm3 of aqueous 35% hydrogen peroxide solution and stirring is carried out to effect mixing. The resulting aqueous solution is then transferred into a 250 cm3 graduated conical flask. The waters from three successive rinses of the flask are introduced into the conical flask and are then made up to 250 cm3 with deionized water. The contents of the conical flask are analysed by ICP/AES (Inductively Coupled Plasma/Atomic Emission Spectroscopy). The lower limit of detection for a sulphur-containing compound, expressed as elemental sulphur (S), is 10 ppm.
Among the products used below in the examples:
The LiF used below as reference sample is obtained from the company Aldrich under the commercial reference 20,364-5 and has a purity of 99.99% by weight.
The PF5 is obtained from the company Ozark-Mahoning (batch DF-17-85) and has a purity of greater than or equal to 99%.
The SO2 is obtained from the company L""Air Liquide and has a purity of 99.9% by weight.
The commercial sample of LiPF6, analysed below for comparative purposes, is obtained from the company Aldrich under the reference 45,022-7 (1996-1997 catalogue).